Objective. Drug-drug interactions can limit the safety of colchicine for treating rheumatic diseases. Seven separate drug-drug interaction (DDI) studies were performed to elucidate the in vivo effects of concomitant treatment with colchicine and known inhibitors of cytochrome P450 3A4 (CYP3A4)/P-glycoprotein (cyclosporine, ketoconazole, ritonavir, clarithromycin, azithromycin, verapamil ER [extended release]), and diltiazem ER) on the pharmacokinetics of colchicine. The objective was to develop colchicine-dosing algorithms with improved safety. Methods.All studies were open-label, nonrandomized, single-center, one-sequence, two-period DDI experiments, using two 0.6-mg doses of colchicine, separated by a minimum 14-day washout period, followed by administration of the approved on-label regimen of known CYP3A4/P-glycoprotein inhibitors. Plasma concentrations of colchicine, but not the reference CYP3A4/P-glycoprotein inhibitors, were determined, and the pharmacokinetic parameters were calculated.Results. The ratios of the maximum concentration and area under the curve from time 0 to infinity for colchicine plus CYP3A4/P-glycoprotein inhibitors versus colchicine alone were >125% across all studies, with the exception of studies involving azithromycin. Significant DDIs were present when single doses of colchicine were coadministered with most of the selected CYP3A4/ P-glycoprotein inhibitors. Recommended colchicine dose reductions of 33-66% for the treatment of acute gout and 50-75% for prophylaxis were calculated for concomitant therapy with each agent, with the exception of no dose adjustment when colchicine is used in combination with azithromycin.Conclusion. These studies provide quantitative evidence regarding drug interactions and necessary adjustments in the dose of colchicine if colchicine treatment is continued during therapy with multiple CYP3A4/Pglycoprotein inhibitors. We demonstrated the need for specific reductions in the dose of colchicine when it is used in combination with 2 broadly prescribed calcium channel blockers (verapamil ER and diltiazem ER) and that the dose of colchicine does not need to be adjusted when it is used in combination with azithromycin.Despite the widespread use of colchicine for the treatment and prophylaxis of gout flares (1-5) and the ClinicalTrials

www.ClinicalTrials.gov identifier NCT00983931 (http://clinicaltrials.gov/ct2/show/NCT00983931).

In contrast to label warnings based on the literature, grapefruit juice did not affect the pharmacokinetics of colchicine. Seville orange juice paradoxically reduced absorption of colchicine and increased T(max), but the clinical significance of this is unknown. Contrary to the expected effects of inhibiting the enzymes that metabolize colchicine, neither juice increased exposure to colchicine. However, the absence of a positive control in these studies dictates that caution should be used when applying these results clinically. ClinicalTrials.gov identifiers: NCT00960193 and NCT00984009.

obJecTive: Review the magnitude and clinical relevance of drug-drug interactions between a new formulation of colchicine, used to treat gout, and antibiotics. seTTinG and PracTice descriPTion: Relevant to community and institutional pharmacists servicing patients with gout.PracTice innovaTion: Pharmacists have clear roles for the identification of drug-drug interactions, providing recommendations for alternative therapy or dose adjustments/modifications, and monitoring for interactionrelated adverse events.abbreviaTions: AE = Adverse event, AUC = Area under the drug concentration-time curve, C max = Maximum plasma concentration, CYP = Cytochrome P450, NSAID = Nonsteroidal anti-inflammatory drug, P-gp = P-glycoprotein efflux transporter.

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One of the numerous regulatory functions of the Food and Drug Administration (FDA) is the evaluation of drug-drug interactions and the determination of appropriate dose adjustments, if necessary, to ensure the safe and effective use of medications. The FDA considers several data sources when determining the significance of drug-drug interactions. The majority of dose adjustment recommendations are based on specific drug-drug interactions studies. The FDA reviews individual patient pharmacokinetic and safety data from drug interaction studies, determines appropriate dose adjustments, and provides recommendations to update the respective product labeling. Sometimes literature references are submitted to the FDA to support dosing recommendations. Determining an appropriate dose adjustment recommendation based on literature reports is a challenge for the FDA due to the lack of individual patient pharmacokinetic or safety data from these studies. Recently, the FDA encountered a challenging regulatory situation when evaluating literature reports to determine the appropriate dose of efavirenz and rifampin. Although numerous studies were found in the literature about this combination, a dosing recommendation cannot be concluded from the reported data. This article reviews the process the FDA used to evaluate literature to support potential dose adjustments for efavirenz when coadministered with rifampin and the challenges encountered during the process.

Colchicine is a substrate for cytochrome 3A4 (CYP3A4) enzyme and P-glycoprotein efflux transporter (P-gp); consequently, concomitant administration with drugs that inhibit these have the potential to cause clinically significant increases in colchicine plasma concentrations and precipitate adverse events. Ritonavir, a protease inhibitor, elicits potent CYP3A4 and P-gp inhibitory activity. In this open-label, nonrandomized, one-sequence, two-period study, 24 healthy volunteers received a single 0.6-mg dose of colchicine alone and together with multiple-dose ritonavir (100 mg twice daily for 4 days) to evaluate drug-drug interactions. Serial blood samples were collected for the determination of colchicine plasma concentrations. Standard pharmacokinetic parameter values were calculated along with 90% confidence intervals (ie, area under the concentration-time curve plasma from time zero to the time of last quantifiable concentration [AUC 0−t and AUC 0−∞ ], maximum drug concentration [C max ]) for colchicine alone and colchicine combined with multiple-dose ritonavir. The mean C max and AUC 0-t were significantly increased (170% and 245%, respectively) when colchicine was coadministered with ritonavir as compared with colchicine alone. Study data confirm the need for a dose adjustment (approximately 50% reduction) when colchicine is coadministered with strong CYP3A/P-gp inhibitors.

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